Yes! Carb Heat is only taught as a procedural item for your checklist, and so that's the only way I've learned it. I've always wanted to understand it, and to know why you need it at different times, regardless of ambient temperature. This is the complete explanation I was hoping to find. Thanks!
After warching multiple private pilot videos on youtube, I now have a clear understanding as to what a carb heat is, and why it is applied. Thanks for a very simple and informative video.
Very useful. There was just one part that I didn't feel was fully explained. At 2:00 you talk about low RPM means the butterfly valve is nearly closed. That's clear enough. But then you say that means the temperature drop "is significant because the Venturi effect is significant." I didn't really feel that was a proper explanation and I'm not really sure what point was being made here. Any clarifications very welcome (by anyone)! Is it saying that ice formation causes the butterfly valve to become rigid and thereby cannot move, so the Venturi effect can't pull in the fuel mixture to be supplied to the engine? Or something else?
Good question. When the butterfly valve is nearly closed, the fuel/air mixture has much less space to squeeze through. This causes the static pressure to further decrease and consequently the temperature to significantly decrease.
@@flightclubonline Aha, so it exacerbates the problem with static pressure cooling at lower engine RPM. Thanks for the quick response! Directly cleared up my confusion! :) What a great channel this is. I'm very glad I found it.
Yeah, I am quite sure use of the term static pressure is incorrect here, although aviators use it more frequently due to analogy to the aircraft’s static pressure system. Let me try to clarify it somewhat and check what I remember from university lectures. Sorry for my language, I am not a native speaker of English. According to Bernoulli’s equation, the fluid’s momentary pressure consists of three elements: its hydrostatic pressure (elevation of the fluid x density x gravitational acceleration = potential element of pressure) and the flow derived element (1/2 x density x linear velocity squared = kinetic element of pressure) and the pressure along the line of flow; if you add these all up you get a constant. What it means is that when you have an airflow in a tube with uniform diameter and there is a section with smaller effective diameter, given the mass flow is constant in a tube, your linear velocity of the fluid increases and it also means a pressure drop at the very same point where the velocity increase happens. Okay, now we have a pressure drop and when this happens, the volume of the compressible fluid (air-gas mixture) increases and this state change is also close to adiabatic because there is not enough time to exchange energy with the environment, therefore this fluid will cool down. It would act exactly like this if it was an ideal gas (ref. ideal gas law), but since we added gasoline into the equation, we have an evaporative cooling effect of that (because gasoline would like to keep its vapor pressure), too, which adds substantially to the temperature drop.
For the longest time I could never understand why the ice formed. Everybody just said "icing" which made me wonder how the heck does such a massive pressure differential freeze water vapor? Then I read somewhere "fuel evaporation icing" and I had a eureka moment. The evaporation rate and cooling effect of Avgas mixed with the water vapor..
There are many advantages to Fuel Injection, such as increased efficiency but carburettors are cheaper to manufacture, make hot engine starts easier and are less prone to issues with fuel contamination.
how is there venturi effect more significant when the throttle is closed? wouldn't the more open the throttle, the more air is allowed to flow through the venturi causing the venturi effect to be more significant?
Vacuum drops as throttle opens, and it's because as the throttle opens, it's reducing the restriction. Vacuum is effectively is a measure of restriction. Venturi relies on vacuum... More vacuum = more pull on Venturi. Modern engines use MAP reading (Manifold Absolute Pressure) as a way to read engine load (high vac aka minus pressure, is light load. Low vac aka almost positive pressure, is heavy load). Vacuum will read around -18" mercury at idle, because a lot of air is being pulled through a tiny "straw". Venturi works on pressure drop principle, so the more pressure drop it gets, the more fluid it can pass through. At WOT (Wide Open Throttle) the "straw" is now equivalent to a Pringle's tube, so all the air passes through with minimum restriction, as a result the vacuum drops to almost 0". The Venturi still provides enough fuel because the lack of pressure is compensated for by a higher volume of air passing by it.
Please be 100 percent accurate, if you are going to teach. The carburetor will not be in the line of sight of a pilot, if on top! That's a lie. 2nd, the reason the carburetor is not on top of the engine is incase there is a fuel leak and that fuel could spill all over a hot engine igniting it, and turning it into a fireball.
